This invention relates to a method and system for detecting when a laser beam has broken through the surface of a workpiece. More specifically, this invention relates to breakthrough detection by sensing changes in the magnetic field corresponding to the breakthrough of a wall of the workpiece by a laser beam.
Lasers are sometimes used for drilling holes in various parts. When using a high-power laser beam to drill a hole through a workpiece, it is useful to know when the beam has broken through the wall of the workpiece. Upon this "breakthrough" occurring, one would turn off the laser. The relative position between the laser and the workpiece could be changed by either moving the workpiece or moving the laser such that another hole could be drilled in the same workpiece. Alternately, another workpiece could be placed in the path of the laser, and it could then be turned on in order to drill a hole in the second workpiece.
Various arrangements have been used for determining when the breakthrough occurs. Some breakthrough detection schemes rely upon having phototransistor-type detectors oriented towards the backside (i.e., side opposite the drill side upon which the laser beam is directed) of the workpiece. When the breakthrough occurs, the detectors sense the light and provide a signal indicating that breakthrough has occurred.
Although the phototransistor-type detectors have been somewhat useful, it may be difficult or impossible to properly position a photo-detector of such a system when drilling holes in parts with complex geometries. For example, problems in positioning of the photo-detector may make this type of technique unsuitable for use on cylinders or tori. Among prior patents related to the laser drilling or laser machining of workpieces such as machine parts in U.S. Pat. No. 4,608,480, issued Aug. 26, 1986, to Bizot et al. That patent discloses control of the operating parameters of a laser by sensing the sounds emitted by the melting of the workpiece.
U.S. Pat. No. 4,504,727, issued Mar. 12, 1985, to Melcher et al., discloses a laser drilling system having feedback control based upon sensing an acoustic signal arising from subjecting the workpiece to a laser.
The Kasner et al. U.S. Pat. No. 4,789,770, issued Dec. 6, 1988, shows an arrangement for controlling depth in a laser drilling system. An optical sensing arrangement is used to detect the depth of the hole which is drilled.
The Inoue U.S. Pat. No. 4,689,467, issued Aug. 25, 1987, shows a laser machining apparatus where a source of plasma is used in combination with the laser. Additionally, infrared rays radiated from the workpiece may be sensed in order to automatically adjust the laser beam.
The Ortiz, Jr. et al. U.S. Pat. No. 4,764,655, issued Aug. 16, 1988, shows a laser materials processing system including an arrangement to determine the beam diameter by imaging techniques.
The Anthony U.S. Pat. No. 4,473,737, issued Sep. 25, 1984, shows a laser drilling technique whereby holes are drilled in a transparent or substantially transparent material (i.e., transparent to the laser radiation) with the hole starting on the side of the workpiece or machine part opposite the side on which the laser beam impinges.
Although various known techniques for controlling laser machining processes have been useful, they have often been subject to one or more several disadvantages. Those techniques for breakthrough detection which rely upon optical sensing at the backside of the drilled part are not suitable for use with workpieces having complex geometries as discussed above. Those optical sensing arrangements which are visually observing the drilling operation from the front or drill side of the workpiece may fail to operate properly because of the various debris which is sprayed outwardly on the drill or front side of the workpiece. Many prior arrangements for controlling laser drilling operation require complex calibration procedures and complex circuitry.